1. Field of the Invention
The present invention relates to fine powder of hydrophobic titanium oxide and to a method for producing it. In particular, it relates to fine powder of hydrophobic titanium oxide which is added as an additional agent to powder coating compositions, toners for electrophotography and other compositions for the purpose of, for example, improving their powdery flowability, preventing them from becoming caked, and controlling the electrification thereof.
2. Description of the Related Art
Many patents have heretofore been issued relating to fine powders of hydrophobic titanium oxide that are used, for example, as additional agents in toners for electrostatic development.
JP-A 5-72797 discloses ultra-fine grains of amorphous and hydrophobic titanium oxide that are produced through vapor-phase pyrolysis or hydrolysis of a volatile titanium compound to give fine grains of amorphous titanium oxide followed by treatment of the grains with an organosilane compound.
JP-A 60-136755 describes a technique of adding both hydrophobic titanium oxide and hydrophobic silica to a negatively charged toner. The hydrophobic titanium oxide used therein is prepared from titanium oxide having a mean grain size of 30 nm and a BET specific surface area of 50 m.sup.2 /g.
JP-A 8-220791, 8-220795 and 8-220796, disclose a technique of using, as an additional agent for toners, hydrophobic titanium oxide as prepared in a wet method and processed in an aqueous system for hydrophobication.
However, among these conventional fine powders of hydrophobic titanium oxide, the ultra-fine grains of hydrophobic amorphous titanium oxide described in JP-A 5-72797 contain a large amount of water which is adsorbed inside the grains and is therefore problematic because their electrification properties often fluctuate depending on changes in the surroundings including temperature and humidity, and they exhibit a weak electrification.
The hydrophobic titanium oxide described in JP-A 60-1367 55, which is prepared from titanium oxide having a mean grain size of 30 nm and a BET specific surface area of 50 m.sup.2 /g, is problematic in that, when it is added to toners by itself, it does not ensure good flowability of toners containing it.
The fine powder of hydrophobic titanium oxide described in JP-A 8-220791, which is prepared in a wet method and is processed in an aqueous system for hydrophobication, is problematic in that it requires a drying step, and that an additional step of pulverizing the aggregates of the powder having a large bulk density is indispensable. In addition, the fine titanium oxide powder produced according to the disclosed method contains a large amount of water adsorbed thereto. Therefore, like that in JP-A 60-136755, the powder is still problematic in that its electrification property often fluctuates depending on changes in the surroundings including temperature and humidity.
Moreover, an additional problem with all of these conventional fine powders of hydrophobic titanium oxide is that they are expensive to produce.
As mentioned above, the conventional fine powders of hydrophobic titanium oxide all face, in no small way, the above-mentioned problems in electrification properties, flowability, and production costs.
On the other hand, a recent tendency in the field of electrophotography is toward producing images of much higher quality, for which the grains to be used are much smaller. The same applies to various powder coating materials. Under these circumstances, the necessity of enhancing the flowability of various powders, such as toners and others, is increasing greatly.